Mr. Udall, who directs a nonprofit energy office in Aspen, Colo., uses alternative energy to power his home. He sells any excess back to his utility for the same price it charges him.

The idea is called net-metering and states are rushing to enact the idea. But even in the more than 25 states where such a system already exists, homeowners should take a close look at the costs.

Sure, you too can make your meter run backward, but the upfront costs can short-circuit any savings.

For example, Udall uses two forms of alternative energy: solar electricity and solar hot-water heating. Of the two, solar hot-water heating is the much better buy, especially in homes like his that use electric instead of gas water heaters.

"Solar hot-water systems are the forgotten stepchild" of alternative energy, he says. His hot-water system has saved as much electricity as his solar-power unit at one-fifth the cost.

Solar power is much harder to justify - at least for homeowners already hooked up to a traditional utility.

Critics of alternative energy wonder if it will ever become economically viable. Udall himself figures his system would have to run 65 years - more than twice its guaranteed life - to pay for itself.

"It's more expensive power, but in my private calculus ... it's worth it to own one of these systems," he says. The reason: By substituting solar energy for conventional power over 20 years, he'll keep some 250,000 pounds of carbon dioxide out of the atmosphere.

That's as much of the greenhouse gas as he'd create driving a car 10 times around the world.

Udall isn't alone. Many consumers seem willing to pay extra for renewable energy. For example: When the municipal utility of Austin, Texas, surveyed its customers four years ago, it found 70 percent were willing to pay more for its renewable-energy projects. "We were amazed by that number," says Leslie Libby, an engineer with solar expertise at Austin Energy.

Its Solar Explorer Program, in which customers volunteer to pay an extra $3.50 a month to support solar energy, is fully subscribed. In Colorado, more than 16,000 people have signed up to support a 21-megawatt wind farm, the state's first.

"People are beginning to differentiate between 'cheap' and 'clean,' " says Udall. His nonprofit in Aspen has begun paying people who install solar-electric systems 25 cents per kilowatt-hour (three times the retail rate) for all the energy their systems produce during the first three years.

This program, the first of its kind in the US, is modeled after successful efforts in 40 Swiss and German cities.